Some of the more serious consequences of periodontal disease include changes in bone structure around teeth. Typically, detecting these changes in bone structure requires the comparison of sequential radiographs recorded over time for the affected areas. These comparisons may be made both visually and by using subtractive methods via computer, but both these methods suffer from the same significant shortcomings which will now be described.
Current dental radiographic equipment and procedures include the use of devices for ensuring proper set-up of the radiographic system. Such devices include beam size rulers and dental beam diameter gauges for measuring beam size; a mesh for determining focus and clarity; and comparators and pinhole cameras for measuring focal spot size. Devices used for measuring the size of features in an X-ray image include radio-opaque rulers and Fixott-Everett grids.
The Fixott-Everett grid is generally placed in contact with the X-ray film during exposure and results in a grid of known size being imaged on the film. Dental rulers are used to measure the size of a feature in an image on the X-ray film. However, these devices and methods alone provide no way to determine if the scale on the film is other than 1:1, nor do they ensure that the scale is consistent between X-rays taken at different times, and with different set-ups. Due to the divergent nature of the X-ray beam and the varying separation between the X-ray source, the tooth, and the film, measurements made directly on the film, whether with a ruler or a grid, fail to account for the attendant scale change.
Consequently, a precise determination of the size of the a feature is not possible from X-ray to X-ray due to the effects on scale caused by the problems described above. While qualitative assessments such as “bigger” or “smaller” are sometimes possible, what is required is the ability to precisely determine the relative increase or decrease in size of a feature between subsequent X-ray images, and the rate of that change.
Thus, there remains a need therefore for an improved method of determining the change in size of features which has taken place between subsequent X-ray images.